The compression pressure which is necessary for securing transformers against short-circuits, which pressure is exercised from the core yokes on the windings, must be transmitted through the insulation between the winding ends and yokes, the so-called yoke insulation. This yoke insulation includes a number of insulating spacers, and generally also insulating collars, for achieving an extension and vision of the spark length between the winding ends and yokes. To hold the collars in position, part of the collar, the so-called collar foot, is inserted between two spacers. When compression pressure is applied, the foot of the collar is firmly attached between two spacers by pressing. The material in the collar may often have poorer yielding and setting properties than the material of the spacer which can be used. Thus, the foot of the collar may become deformed by yielding under the influence of a quite normal compression pressure, which causes the compression pressure on the winding, necessary for securing against short-circuits, to be reduced; and in extreme cases the collars may become detached. This is true not only in the case of pressboard collars, but also for collars made of plastic material (e.g. polyolefins), which may sometimes be desirable to use because of their low dielectric constant.